338 LETTERS ON NATURAL MAGIC. 



and dilates like the pupil of the eye. When the * 

 vaporization is so feeble as to produce only a 

 single ring of one or two tints of the second 

 order, they vanish instantly by breathing upon 

 the crystal; but when the slight heat of the 

 breath reaches the fluid, it throws off fresh 

 vapour, and the rings again appear. 



If a drop of ether is put upon the crystal when 

 the rings are in a state of rapid play, the cold 

 produced by its evaporation causes them to dis- 

 appear, till the temperature again rises. When 

 the temperature is perfectly uniform, the rings 

 are stationary, as shown between V and V in 

 fig. 84 ; and it is interesting to observe the first 

 ring produced by the vapour swelling out to 

 meet the first ring at the margin of the fluid, 

 and sometimes coming so near it that the darkest 

 parts of both form a broad black band. As the 

 heat increases, the vacuity V V diminishes and 

 disappears at 79, exhibiting many curious phe- 

 nomena, which we have not room to describe. 



Having fallen upon a method of opening the 

 cavities, and looking at the fluids, I was able to 

 examine their properties with more attention. 

 When the expansible fluid first rises from the 

 cavity upon the surface of the topaz, it neither 

 remains still like the fixed oils, nor disappears 

 like evaporable fluids. Under the influence, no 

 doubt, of heat and moisture, it is in a state of 

 constant motion, now spreading itself on a thin 

 plate over a large surface, and now contracting 

 itself into a deeper and much less extended drop. 

 These contractions and extensions are marked 

 by very beautiful optical phenomena. When 

 the fluid has stretched itself out into a thin 



